1. Technical Field
This invention concerns a process for controlling the operation of a print head that is moved from the printing position in a given direction and within a given transport time to another position after printing a character by actuation of at least one printing element, whereby the transport time is monitored to determine whether a limit value is exceeded, and if the limit value is exceeded, the drive of the print head is interrupted.
2. Background Art
Processes of this type are used in printers for printing text, data or graphic information. They are used to bring a print head into a given printing position and then produce an impression on a substrate such as paper by actuation of at least one printing element. To do so, an ink ribbon is arranged between the print head and the substrate and comes in contact with the substrate when the printing element is actuated. With a number of printers, the information to be printed is printed character by character, i.e., the print head is shifted in the direction of the line and in doing so executes the printing operation.
After printing a line, the printed substrate is advanced by the distance equal to one line by means of a roller, for example, and then the line-by-line printing operation is carried out again. During printing, the instantaneous position of the print head in a line is detected and can be relayed by means of a position-reporting signal to a controller that actuates a printing element to print a character on reaching a certain position. This principle can be used in an interval-controlled operation, whereby the movement of the print head is interrupted and then the printing element is actuated, and it can also be used in so-called "flying" printing. In the latter case, the print head is not stationary when printing a character but instead the printing element is actuated without interrupting the movement of the print head along the line.
In operation of a print head, the print head may adhere to the printing medium, i.e., the ink ribbon or the substrate, after actuating a printing element. For example, with a needle print head, whose printing elements are needles, a needle actuated for printing can puncture the ink ribbon and hook into it so that it does not return to its resting position. When the print head is advanced to the next printing position, the needle can be either bent or broken off due to the relative motion between the needle and the ink ribbon. This causes destruction of the entire print head. This risk is increased with recent needle printers due to the fact that the number of needles per print head is high and thin needles that can more easily puncture the ink ribbon are used. Even with printing processes that do not use an ink ribbon but use so-called active paper as the substrate, it is still possible for a printing element to penetrate into the substrate and adhere to it by hooking into it.
In other printing processes, it is also possible for the print head and ink ribbon to stick together. For example, in heat transfer printing, where the printing elements are electric heating resistors that transfer the character to the printing stock with the help of a melting ink ribbon, there is the danger that the printing element of the print head might stick to the melting tool adhering to the ink ribbon. The ink ribbon, which is made of a plastic film, may also adhere to the printing elements due to overheating or because of surface defects, which thus can cause interference in operation of the print head.
In addition, interference can also occur in transport of the print head due to an incorrectly inserted printing stock, due to a paper jam or due to trouble in the drive system. In all these cases, it would be desirable to be able to detect these problems promptly in order to prevent damage to sensitive components.